1. Field of the Invention
The present invention relates to a semiconductor module, a method for manufacturing the semiconductor modules, and a mobile device.
2. Description of the Related Art
There is a type of semiconductor module called a CSP (Chip Size Package) among the conventional semiconductor modules. A semiconductor module of CSP type is produced by a process in which a semiconductor wafer (semiconductor substrate) with LSIs (semiconductor devices) and external connection electrodes connected thereto formed on a main face thereof is diced into individual modules. Therefore, a semiconductor module can be fixed onto a wiring substrate in a size practically the same as an LSI chip. This helps realize the miniaturization of a wiring substrate on which the semiconductor module is mounted.
In recent years, following the trend of electronic devices towards miniaturization and higher performance, demand has been ever greater for further miniaturization of semiconductor modules which are used in them. To realize such miniaturization of semiconductor modules, it is of absolute necessity that the pitch of electrodes that allow packaging on wiring substrate be made narrower. A known method of surface-mounting a semiconductor device is flip chip mounting in which solder bumps are formed on external connection electrodes of the semiconductor device and the solder bumps are soldered to an electrode pad of a wiring substrate. With this flip-chip mounting method, however, there are restrictive factors for the narrowing of the pitch of external connection electrodes, such as the size of the solder bump itself and the bridge formation at soldering. A way used to overcome these limitations in recent years has been a rearrangement of external connection electrodes by forming a rewiring in the semiconductor device. For example, in a known method for such rearrangement, a bump structure formed by half-etching a metal sheet is used as an electrode or a via, and external connection electrodes of the semiconductor device are connected to the bump structure by mounting the semiconductor device on the metal sheet through an insulating layer of an epoxy resin or the like.
On the other hand, in order to improve connection reliability, between the electrode made of metal and the bump structure, in manufacturing flexible substrates and printed boards, a method has been proposed where an adhesion layer such as flexible metallic coating (e.g., gold film) or conductive adhesive (e.g., soldering paste) is provided at a tip or peripheral part of the bump structure and a connection is made between the electrode and the bump structure through this adhesion layer.
However, differing from the connections found in the flexible substrates and printed boards, thermal expansion coefficients differs vastly between silicon constituting a semiconductor device and the metal sheet such as copper in the case of a semiconductor module. For this reason, heat stress due to a difference in thermal expansion coefficients between materials occurs at a connection portion, where the external connection electrode and the bump are connected together, as a result of a temperature change caused by a heat treatment or the like. In particular, when in the future the miniaturization of external connection electrodes and connection pitch of the bump structure advances to realize further reduction in size of the semiconductor modules, a contact area between the external connection electrode and the bump structure becomes smaller. As a result, there is concern that such heat stress may cause disconnection in the connection between the external electrode and the bump structure.